Hydroxycarboxylic acids and hydroxycarboxylic acid salts are well recognized as corrosion inhibitors particularly effective in inhibiting metal corrosion when the metal is in contact with water or an aqueous solution. U.S. Pat. No. 2,529,178 to Nieland et al. taught that these hydroxycarboxylic acids, or salts thereof, may contain a single carboxylic acid function, as in the case of gluconic acid (U.S. Pat. No. 2,529,178), or more than one carboxylic acid functions as in the case of tartaric acid, a hydroxydicarboxylic acid, or citric acid, a hydroxy tricarboxylic acid (U.S. Pat. No. 2,529,177). Nieland et al. also taught that hydroxycarboxylic acids, or salts thereof, with more than one carboxylic acid functions, such as tartaric acid (U.S. Pat. No. 2,529,177), generally exhibit better corrosion inhibition properties than do comparable hydroxymonocarboxylic acids, such as gluconic acid (U.S. Pat. No. 2,529,178).
Hydroxycarboxylic acids have also been shown to inhibit metal corrosion in aqueous salt brine such as sea water (Mor, 1971; Mor, 1976; and Wrubl, 1984) or formulated brine solutions (Kuczynski, 1979; Korzh, 1981; Sukhotin, 1982; and Abdallah, 1999), and some are employed for specific applications, such as in industrial cooling systems (Sukhotin, 1982).
In addition, hydroxycarboxylic acids and hydroxycarboxylic acid salts have been described as chelating agents capable of sequestering metal ions in solution (Mehltretter, 1953; Abbadi, 1999). Hydroxycarboxylic acid salts as sequestering agents for metal ions such as calcium and magnesium, in general perform poorly compared to common sequestering agents such as sodium tripolyphosphate (STPP), ethylenediaminetriacetate (EDTA), or nitrilotriacetate (NTA). In spite of low sequestering capacity, hydroxycarboxylic acid salts are of interest because they are typically biodegradable, non-toxic, and derived from renewable resources such as carbohydrates. Therefore, the use hydroxycarboxylic acid salts as replacement sequestering agents for STPP and EDTA is advantageous, especially in applications where the compounds may be discharged into the environment.
Many chemical compounds that have traditionally been used as corrosion inhibitors and metal sequestering agents are phosphorus based. Through environmental regulations, the use of phosphorus compounds in applications where the material is discharged into surface water continues to be restricted. These regulations have created a need for environmentally acceptable materials for use as corrosion inhibiting agents for a variety of applications. One area of specific need centers around corrosion inhibiting agents which are also capable of sequestering metal ions, specifically metal ions like calcium and magnesium commonly found in tap water or fresh ground water and which can lead to scale formation. In particular, agents capable of both preventing corrosion and inhibiting scale would be useful as builders in detergents or as additives to process water used in industrial cooling towers and boiler systems.
One application in which corrosion inhibitors which act as sequestering agents are useful is in detergent formulations. Detergents are cleaning mixtures composed primarily of surfactants, builders, bleaching-agents, enzymes, and fillers. Two of the major components are surfactants and builders. The surfactants are responsible for emulsification of oil and grease while builders are added to extend or improve the cleaning properties of the surfactant. The builder can be a single substance or a mixture of substances and commonly serve multiple functions. An important builder function is the sequestration of metal cations, typically calcium and magnesium cations in hard water. The builders act as water softening agents by sequestering calcium and magnesium cations and preventing the formation of water insoluble salts between the metals and surfactants (soap scum). In the case of laundry detergents, builders also help prevent the cations from binding to cotton, a major cause of soil retention on cotton fabrics. Other functions of builders include increasing alkalinity of detergent solutions, deflocculating surfactant micelles, and inhibiting corrosion.
The first builders used in commercial detergents were phosphate salts and phosphate salt derivatives. Sodium tripolyphosphate (STPP) was, at one time, the most common builder in both consumer and industrial detergents. Phosphate builders are also touted as corrosion inhibitors for the metal surfaces of washing machines and dishwashers. Phosphates have been gradually phased out of detergents over the past 40 years primarily due to environmental concerns regarding discharge of phosphate rich waste water into surface waters giving rise to eutrophication and ultimately hypoxia (Lowe, 1978). High performance replacements for phosphates in detergents are still sought after.
A second application area for corrosion inhibitors which serve as sequestering agents is in the treatment of process water used in industrial and institutional cooling tower and boiler systems. Two of the primary problems associated with cooling tower and boiler systems are corrosion and build-up of hard water scale on metal surfaces. Corrosion shortens the life of the system equipment while scaling decreases the efficiency of heat flow across the metal surfaces. Historically, phosphates and phosphonates have been used in water treatment as corrosion and scale inhibitors. As with phosphate detergent builders, there is a desire to replace phosphorus-based chemicals used in water treatment. Accordingly, a need exists to have a corrosion inhibitor which also serves as a sequestering agent, and which does not incorporate phosphates or chemicals that have been deemed harmful to the environment.